I. Field of the Invention
This invention relates to improved start-up operations with ruthenium catalysts, especially ruthenium catalysts such as those used in Fischer-Tropsch synthesis, or in methanol conversion to produce hydrocarbons.
II. The Prior Art
Fischer-Tropsch synthesis for the production of hydrocarbons has been known for many years. The use of ruthenium as a catalyst for the production of high-melting hydrocarbon wax from carbon monoxide and hydrogen has been known since the late thirties or early forties. Ruthenium is in limited supply, but on the positive side, ruthenium is known as one of the more active catalysts for use in Fischer-Tropsch synthesis, and its selectivity for making methane in the production of hydrocarbons is relatively low. Moreover, it is recognized as having a low carbon dioxide selectivity. The ruthenium catalyst thus behaves somewhat more ideally than many other catalysts, e.g. iron catalysts, in that more of the hydrogen and carbon monoxide of a synthesis gas are converted to hydrocarbons and water in accordance with the idealized equation: 2H.sub.2 +CO.fwdarw.(CH.sub.2).sub.x +H.sub.2 O; with less of the synthesis gas being converted to carbon dioxide, as in the equation: H.sub.2 +2CO.fwdarw.(CH.sub.2).sub.x +CO.sub.2. The low carbon dioxide selectivity makes use of a ruthenium catalyst for the production of hydrocarbons particularly advantageous for use in processing synthesis gas derived by the conventional technique of steam reforming light hydrocarbon gases, e.g. refinery gas and natural gas. More recently, it has been discovered that ruthenium catalysts are useful for the conversion of methanol to hydrocarbons.
In Exxon Research and Engineering Co.'s U.S. Pat. No. 4,042,614 to Vannice et al which issued Aug. 16, 1977, there is disclosed a ruthenium catalyst, the ruthenium being dispersed on TiO.sub.2, other titanium-containing oxides or mixtures of titanium oxides, which provides superior synthesis characteristics in the conversion of carbon monoxide and hydrogen to hydrocarbons, notably olefinic hydrocarbons, particularly C.sub.2 to C.sub.10 olefins. These catalysts, like other ruthenium catalysts, have low methane selectivity, high activity, and low carbon dioxide selectivity. They are also suggested by Vannice et al as having, when treated by contact with air at about 100.degree.-150.degree. C., improved longevity and tolerance to sulfur, and resistance to volatilization in oxidizing atmospheres as contrasted with prior art ruthenium catalysts wherein the ruthenium is supported on other materials, e.g., Al.sub.2 O.sub.3, SiO.sub.2, carbon or the like.
The synthesis of hydrocarbons from carbon monoxide and hydrogen, and conversion of methanol to hydrocarbons over ruthenium catalysts are highly exothermic reactions. Ruthenium-titania catalysts are very active and are capable of providing high conversion at high space velocities with low methane yields. It is essential, however, to temper the extremely high activity exhibited by fresh catalyst, and thereby control the large heat release which leads to high methanation, not only to avoid loss of selectivity in providing the more desirable higher molecular weight hydrocarbons, but also to avoid damage to the catalyst, and to the reactor and auxiliary equipment. Inevitably, to control this heat release on a fresh catalyst, it appears necessary to conduct the synthesis and methanol conversion reactions over a long break-in, or start-up period at low severities. This period generally lasts for several days, during which time the selectivity of the catalyst in producing the more desirable higher molecular weight hydrocarbons is less than optimum. Gradually, over the start-up period however, the selectivity in producing the more desirable higher molecular weight hydrocarbons improves, the severity of the reactions can be gradually increased, and eventually the product stream is optimized in terms of the desired product. The reactor, in terms of production, thus lines out. However, optimization of product yield, requires a long time period with concurrent loss of production. Attempts to cut short this catalyst break-in or start-up period have led to uncontrolled exotherms and eventual catalyst destruction.